How to draw an irregular closed shaped in LaTeX

Question

I have the following code in LaTeX that produces a circle in a square with arrow lines, but I want to replace the circle with an irregular shape like the molecule in the image. How can I go about it?

\begin{figure}[H]
\centering
\begin{tikzpicture}[thick]
  %x-axes
  \draw [red](4,2) --(7,2);
  %\draw (4,3) --(7,3);
  %\draw (4,4) --(7,4);
  \draw [red,very thick](4,5) --(7,5);
  %y-axes
  \draw [red,very thick](7,2) --(7,5);
  %\draw (6,2) --(6,5);
  %\draw (5,2) --(5,5);
  \draw [red,very thick](4,2) --(4,5);
  %centre
  \fill[blue,very thick] (5.5,3.5) circle(.1);
  %arrows on the right
  \draw [-latex][dashed] (5.5,3.5) --(8,5);
  \draw [-latex][dashed] (5.5,3.5) --(8,4);
  \draw [-latex][dashed] (5.5,3.5) --(8,3);
  \draw [-latex][dashed] (5.5,3.5) --(8,2);
   %arrows on the left
  \draw [-latex][dashed] (5.5,3.5) --(3,2);
  \draw [-latex][dashed] (5.5,3.5) --(3,3);
  \draw [-latex][dashed] (5.5,3.5) --(3,4);
  \draw [-latex][dashed] (5.5,3.5) --(3,5);
  %arrows at the bottom
  \draw [-latex][dashed] (5.5,3.5) --(4,1);
  \draw [-latex][dashed] (5.5,3.5) --(5,1);
  \draw [-latex][dashed] (5.5,3.5) --(6,1);
  \draw [-latex][dashed] (5.5,3.5) --(7,1);
  %arrows at the top
  \draw [-latex][dashed] (5.5,3.5) --(4,6);
  \draw [-latex][dashed] (5.5,3.5) --(5,6);
  \draw [-latex][dashed] (5.5,3.5) --(6,6);
  \draw [-latex][dashed] (5.5,3.5) --(7,6);
  %draw a circle
  \node(circle) [circle, inner sep=0.7cm, draw=red!120] at (5.5,3.5) {};
\end{tikzpicture}
\end{figure}

Answer 1

This is what you can do with the hobby package:

\documentclass[12pt]{article}
\usepackage{tikz}
\usetikzlibrary{hobby}

\begin{document}
\begin{tikzpicture}[scale=2]
  \draw [red,very thick](4,2) --(7,2);
  \draw [red,very thick](4,5) --(7,5);
  \draw [red,very thick](7,2) --(7,5);
  \draw [red,very thick](4,2) --(4,5);

  \fill[blue,very thick] (5.5,3.5) circle(.1);
  %arrows on the right
  \draw [-latex][dashed] (5.5,3.5) --(8,5);
  \draw [-latex][dashed] (5.5,3.5) --(8,4);
  \draw [-latex][dashed] (5.5,3.5) --(8,3);
  \draw [-latex][dashed] (5.5,3.5) --(8,2);
   %arrows on the left
  \draw [-latex][dashed] (5.5,3.5) --(3,2);
  \draw [-latex][dashed] (5.5,3.5) --(3,3);
  \draw [-latex][dashed] (5.5,3.5) --(3,4);
  \draw [-latex][dashed] (5.5,3.5) --(3,5);
  %arrows at the bottom
  \draw [-latex][dashed] (5.5,3.5) --(4,1);
  \draw [-latex][dashed] (5.5,3.5) --(5,1);
  \draw [-latex][dashed] (5.5,3.5) --(6,1);
  \draw [-latex][dashed] (5.5,3.5) --(7,1);
  %arrows at the top
  \draw [-latex][dashed] (5.5,3.5) --(4,6);
  \draw [-latex][dashed] (5.5,3.5) --(5,6);
  \draw [-latex][dashed] (5.5,3.5) --(6,6);
  \draw [-latex][dashed] (5.5,3.5) --(7,6);
  %draw a circle
  \node(circle) [circle, inner sep=0.7cm, draw=red!120] at (5.5,3.5) {};

  \coordinate[label=below left:{$A$}] (A) at (4+0.8,2+0.8);
  \coordinate[label=left:{$B$}]       (B) at (4+0.4,2+1.5);
  \coordinate[label=above:{$C$}]      (C) at (4+0.5,2+2.5);
  \coordinate[label=above:{$D$}]      (D) at (4+1.0,2+2.0);
  \coordinate[label=below:{$E$}]      (E) at (4+1.8,2+2.7);
  \coordinate[label=below left:{$F$}] (F) at (4+2.4,2+2.4);
  \coordinate[label=left:{$G$}]       (G) at (4+2.7,2+1.8);
  \coordinate[label=right:{$H$}]      (H) at (4+2.0,2+1.0);
  \coordinate[label=right:{$I$}]      (I) at (4+2.5,2+0.5);
  \coordinate[label=below left:{$J$}] (J) at (4+1.5,2+0.4);

  \draw[blue,very thick] (A) to [closed, curve through = {(B) (C) (D) (E) (F) (G) (H) (I) (J)}] (A);

  \foreach \j in {A,...,J}{\fill[blue] (\j) circle (0.04);}

\end{tikzpicture}

\end{document}

Answer 2

As your MWE had a strange sketch not so related with the result you wanted, I got a little bit confused, sorry. Anyhow, I will present:

1. A redraw of your original code, as a base of how some commands work.
2. A colorful draw of your desired result.
3. Another method to draw a smooth curve, from which you can choose which one to use.

I let colors in order to make it easier to identify who does what, but not exactly the ones you presented. You may mix and edit code at your will.

MWE 01

Please, notice how much simpler are some the code when some commands are used. I invite you check A very minimal introduction to TikZ.

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[thick]
  \draw [red,very thick] (4,2) rectangle ++(4,4);
  \fill[blue,very thick] (6,4) coordinate (A) circle (0.2); 
  \foreach \t in {0,20, ..., 360}{
    \draw [-latex, dashed]  (A) -- ++(\t:3.5);
  };
  \node(circle) [circle, inner sep=0.7cm, draw=red!120] at (A) {};
\end{tikzpicture}
\end{document}

MWE 02

The main MWE. The closed smooth parth is drawn with ... plot[smooth cycle, tension=.7] coordinates {...};. Notice how scope compacts the code to applied every node, it makes the code much more clean and clear to read.

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[thick]
  \draw [magenta,very thick] (0,0) node[left]{Solvent} rectangle ++(6.8,5.5);
  
  \draw[green!50!black]  plot[smooth cycle, tension=.7]
  coordinates {(0.8,0.6) (3.3,1.2) (3.3,2.5) (4.4,2.9) (5.5,3.5) (4.8,5.1) (1.3,3.6)};
  
  \node at (7.2,0.1) {$\partial \Omega$};
  \node[green!50!black] at (5.6,5.1) {$\Gamma$};
  \node[green!50!black] at (2.9,5) {$\Omega^{+}$};
  \node[green!50!black] at (2.9,4.1) {$\Omega^{-}$};
  \node[green!50!black] at (2.9,3.1) {Molecule};
  \node[orange] (B) at (5.5,1) {Mobile Ions};
  
  % Base syntax
  % \node[circle, red, inner sep=0.1cm, draw] (circle) at (4.1,-1.3) {$+$};
  % \node[circle, blue, inner sep=0.1cm, draw] (circle) at (2.4,-2.9) {$-$};
  
  \begin{scope}[every node/.style={circle, red, inner sep=0.1cm, draw}]
    \node at (3.7,3.8) {$+$};
    \node at (2,2.2) {$+$};
    \node at (1.7,3.1) {$+$};
  \end{scope}
  
  \begin{scope}[every node/.style={circle, blue, inner sep=0.1cm, draw}]
    \node at (4.6,4.4) {$-$};
    \node at (1.3,1.4) {$-$};
    \node at (1.2,2.3) {$-$};
  \end{scope}
  
  % Base syntax
  % \node[red] (circle) at (6.6,-0.8) {$+$};
  % \node[blue] (circle) at (2,-0.3) {$-$};
  
  \begin{scope}[every node/.style={cyan}]
    \node (P) at (4.7,2.2) {$+$};
    \node at (0.2,1.8) {$+$};
    \node at (0.6,3.8) {$+$};
  \end{scope}
  
  \begin{scope}[every node/.style={teal}]
    \node (N) at (6.1,4.1) {$-$};
    \node at (1.6,4.6) {$-$};
    \node at (0.6,4.6) {$-$};
  \end{scope}
  
  \draw[-latex] (B) -- (N);
  \draw[-latex] (B) -- (P);
\end{tikzpicture}
\end{document}

MWE 03

The alternative to plot[smooth] is using Bezier curves with (A) .. controls (B) and (C) .., where (A) , (B) and (C) are given coordinates.

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  \draw[purple] (9.8,2.5) .. controls (10.8,2) and (12.3,2) .. (11.3,3) .. controls (10.8,3.5) and (12.3,3) ..
  (12.8,4) .. controls (13.3,4.5) and (10,5.4) ..
  (8.8,4.5) .. controls (7.3,3.5) .. cycle;
\end{tikzpicture}
\end{document}