def _surface(self, T):
        """Generic equation for the surface tension

        Parameters
        ----------
        T : float
            Temperature, [K]

        Returns
        -------
        σ : float
            Surface tension, [N/m]

        Notes
        -----
        Need a _surf dict in the derived class with the parameters keys:
            sigma: coefficient
            exp: exponent
        """
        tau = 1-T/self.Tc
        sigma = 0
        for n, t in zip(self._surf["sigma"], self._surf["exp"]):
            sigma += n*tau**t
        return sigma
Generic equation for the surface tension

        Parameters
        ----------
        T : float
            Temperature, [K]

        Returns
        -------
        σ : float
            Surface tension, [N/m]

        Notes
        -----
        Need a _surf dict in the derived class with the parameters keys:
            sigma: coefficient
            exp: exponent